1. Field of the Invention
The present invention relates to a telescopic zoom lens system having a zoom ratio of approximately 4, for use in a single lens reflex camera having a long back focal distance.
2. Description of the Prior Art
As an example of a telescopic zoom lens system for a single lens reflex camera having a zoom ratio of approximately 4, there is known, for example, Japanese Unexamined Patent Publications (JUPP) No. Hei-8-160301 and U.S. Pat. No. 6,052,235 which disclose a zoom lens system of a four-lens-group arrangement; however the overall length of the zoom lens system disclosed in each publication is relatively longer, and is not compact. Furthermore, in JUPP No. Hei-8-160301, the number of lens elements is 14; and in JUPP No. Hei-10-148758, the number of lens elements is 12 to 14. Accordingly, in these publications, the number of lens elements is relatively larger, so that cost reduction becomes difficult.
In a zoom lens system disclosed in JUPP No. 2001-188169, the number lens elements is about 11, which is relatively few; however, the zoom ratio is about 3, and the overall length of the zoom lens system is not short.
The present invention provides a telescopic zoom lens system for a single lens reflex camera, having a relatively short overall length, and being produced at low cost. Furthermore, the present invention provides a telescopic zoom lens system, having a zoom ratio of approximately 4, and including a smaller number of lens elements.
According to an aspect of the present invention, there is provided a zoom lens system including a positive first lens group, a negative second lens group, and a positive third lens group, in this order from the object. Upon zooming from the short focal length extremity to the long focal length extremity, all of the positive first lens group, the negative second lens group and the positive third lens group are moved. The distance between the positive first lens group and the negative second lens group increases, the distance between the negative second lens group and the positive third lens group decreases. Each of the positive first lens group, the negative second and positive third lens group is positioned closest to the object at the long focal length extremity.
The positive third lens group includes a positive first sub-lens group and a negative second sub-lens group, in this order from the object. More specifically, the positive first sub-lens group is positioned at the object-side end of a reference distance which is defined as the longest distance formed between lens elements of said positive third lens group at the short focal length extremity, and the negative second sub-lens group is positioned at the image-side end of the reference distance.
In addition, the positive first sub-lens group is provided with at least one aspherical surface.
The zoom lens system satisfies the following conditions:
1.7 less than fw/f3 less than 2.5xe2x80x83xe2x80x83(1)
xe2x88x920.5 less than fw/f3b less than xe2x88x920.2xe2x80x83xe2x80x83(2)
xe2x88x9245 less than xcex94Iasp less than xe2x88x9220xe2x80x83xe2x80x83(3)
wherein
fw designates the focal length of the entire zoom lens system at the short focal length extremity;
f3 designates the focal length of the third lens group at the short focal length extremity;
f3b designates the focal length of the second sub-lens group at the short focal length extremity; and
xcex94Iasp designates the aberration coefficients of the aspherical-surface terms with respect to the third-order aberration coefficients of the aspherical surface under the condition that the focal length of the entire lens system at the short focal length extremity is converted to 1.0.
In regard to the positive first sub-lens group and the negative second sub-lens group, it is possible to set a state where a relative position therebetween is fixed, and a state where these sub-lens groups are relatively moved upon zooming. The above-described xe2x80x9cthe longest distance between lens elements in the positive third lens group at the short focal length extremityxe2x80x9d means a concept when the positive first sub-lens group and the negative second sub-lens group are relatively moved. On the other hand, in the case where these lens groups are not relatively moved, the above definition is simply read as xe2x80x9cthe longest distance between lens elements in the positive third lens groupxe2x80x9d.
If an attempt is made to employ an aspherical surface in the positive first sub-lens group of the positive third lens group, the correcting of spherical aberration can effectively be made, and the number of lens elements can be reduced. Consequently, production costs can be reduced. Furthermore, in addition to achieving the fewer number of lens elements due to the aspherical surface, if the positive third lens group is constituted as a telephoto-type lens group so that the positive third lens group satisfies condition (2), the overall length of the entire zoom lens system can be made shorter.
The aspherical surface can be formed on a glass lens element or a plastic lens element. On the other hand, the aspherical surface can be formed as a hybrid type in which the aspherical surface is formed on a transparent resin material layer attached on a spherical glass lens element.
In the case where the aspherical surface is formed on a plastic lens element, the aspherical surface preferably satisfies the following condition:
0 less than fw/fp less than 0.2xe2x80x83xe2x80x83(4)
wherein
fp designates the focal length of the plastic aspherical lens element in the positive first sub-lens group.
The positive first lens group preferably includes a positive lens element, a negative lens element, and a positive lens element, in this order from the object.
The negative second lens group can include a negative lens element, a positive lens element, and a negative lens element, in this order from the object.
The first sub-lens group preferably includes a positive aspherical lens element made of glass or a hybrid-type aspherical lens element, a positive lens element and a negative lens element, in this order from the object.
The second sub-lens group preferably includes a positive lens element, and a negative lens element, in this order from the object.
The entire zoom lens system includes eleven lens elements in total.
The positive first lens group preferably includes a positive lens element, a negative lens element, and a positive lens element, in this order from the object.
The negative second lens group can include a negative lens element, a positive lens element, and a negative lens element, in this order from the object.
The first sub-lens group preferably includes a positive lens element, a positive lens element, a negative lens element, and a positive aspherical lens element made of plastic, in this order from the object.
The second sub-lens group preferably includes a positive lens element, and a negative lens element, in this order from the object.
The entire zoom lens system includes twelve lens elements in total.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2002-135738 (filed on May 10, 2002) which is expressly incorporated herein in its entirety.